The invention relates to a heart pacemaker system comprising at least one active electrode and one passive electrode, in particular the heart pacemaker housing. Such heart pacemaker systems are employed, among other things for bifocal stimulation and have, for example, an active electrode which is introduced after the implantation of the heart pacemaker into the atrium of the heart and a further active electrode that is inserted into the ventricle of the heart. Essentially two demands are made of implantable stimulating electrodes which generally are comprised of an insulated conductor and an electrode head connected to said conductor and having the active region:
(1) The electrode material must be compatible with the body so that the formation of connective tissue layers having a thickness greater than one hundred microns (100 .mu.m) is suppressed so that the stimulation threshold remains largely constant.
(2) A high double layer capacitance should develop at the phase boundary electrode/body fluid so that the polarization rise during the stimulation pulses (0.5 through 1 ms, 1 Hz, 10 mA, 10 mm.sup.2) remains less than 0.1 V.
These demands are met to a particularly high degree by electrodes wherein the active region consists of glassy carbon (v. German published application 2613072). A high double layer capacitance of up to 0.1 F/cm.sup.2 is achieved due to an activation of the surface of the glassy carbon.
The only demand hitherto made of the passive electrode was that of compatibility with the body.
Given the said heart pacemaker system comprising two active electrodes and a shared passive electrode, problems can occur under certain conditions due to interactions between the electrodes. The cause thereof is a polarization rise at the passive electrode given stimulation with an active electrode. This polarization is slow to be dismantled and negatively influences the possibility of utilizing the other active electrode for detecting heart depolarizations during this time, since the polarization represents an increase of the electrochemical impedance of the system that makes the detection of the extremely small measurement currents considerably more difficult.
The functionability of the heart pacemaker system, further, can be negatively influenced by muscular convulsions. These muscle convulsions are generally based on the fact that the stimulation pulses not only stimulate the heart muscle but also stimulate stimulatable tissue in the proximity of the heart pacemaker housing which represents the passive electrode in the electrode system. The electrical voltage pulses associated with these muscle convulsions can simulate nonexistent heart activities under certain conditions. Previous attempts to eliminate this danger have been undertaken in that the heart pacemaker was surrounded by an insulating jacket. A hole through which the current can pass is situated in said jacket (SIEMENS-ELEMA brochure ME 372/5406.101, 1979). The heart pacemaker is then implanted such that the hole is situated at the side facing away from the stimulatable tissue. As a rule, the muscle convulsions stop.
In addition to these muscle convulsions, the problem also existed that the heart pacemaker could detect myopotentials of the skeletal musculature in the proximity of the heart pacemaker as depolarizations of the heart musculature. The danger of a negative influence on the heart pacemaker function as a result thereof also existed. This problem was also resolved by means of the insulating jacket and the proper placement of the heart pacemaker upon implantation.
This insulating jacket, however, also presents problems. First, the surface of the passive electrode is reduced, whereby the polarization effects increase and the efficacy of the stimulation pulses and the sensitivity of the heart pacemaker for detecting heart activities are reduced. On the other hand, problems regarding manufacture, hygiene and reliability arise due to this insulating jacket usually consisting of some kind or organic material.